In one widely used architecture, the circuit components of an electronic device are supported on a printed wiring board (PWB) or printed circuit board. The components include, for example, microelectronic devices, packages, and electrically conductive traces. These circuit components may be fragile and/or easily damaged by environmental agents such as chemicals, dirt, and contact with foreign objects. The circuit components are therefore protected from such damage.
One protective approach is a conformal coating. The conformal coating is typically a curable polymer that is applied as an uncured liquid to the individual circuit component or to the entire PWB by dipping, spraying, brushing, screening, or other approach, and thereafter cured. The cured conformal coating encapsulates and seals the coated structure against intrusion of environmental agents, and also provides an insulating barrier between the components and exterior shorts. A commonly used conformal coating is polyurethane.
Another potential source of damage to electronic structures used in spacecraft is the accumulation of electronic charge on the surfaces of the structures. The space environment has a flux of energetic electrons from the solar wind and other sources. These electrons may penetrate spacecraft shielding and accumulate on the surfaces of the electronic devices as static charges. When the static charges accumulate to the extent that they become sufficiently high in voltage, they may discharge uncontrollably by arcing and cause damage to the electronic structure.
Preferably, to protect against such uncontrolled discharge events, the conducting surfaces of electronic components should be grounded by leads extending to a common ground. As for the non-conductive surfaces of electronic components, the ideal solution is to cover these surfaces with a grounded and conductive coating. However, the conductive coating must have the proper electrical conductivity so that the coating does not cause excessive current to flow between electronic components leading to degradation in performance of circuits, or in the extreme case, shorting of circuits. It may be difficult to ground all of the electronic structures, and it is almost always quite expensive to do so.
In an alternative approach disclosed in U.S. Pat. No. 5,160,374, about 60 percent by volume of electrically conductive tin oxide particles are mixed into a resin material to form a paint. The paint is applied over the electronic structures as a conformal coating. The paint has an electrical resistivity of about 106-1012 ohm-centimeter. The paint thus has some electrical conductivity, so that static charge is drained away, but also serves as a semi-insulator. The approach of the ""374 patent is operable to some degree, but the present inventors have recognized that it has significant drawbacks in many applications. First, the resistivity is too low leading to excessive leakage current. Second, the properties of the paint are dominated by the high volume fraction of the particles, so that the mechanical integrity and protective quality of the conformal coating are compromised. The particles may form a shorting bridge between closely spaced elements of the electronic structure, such as circuit traces on a die. Further, the paint is largely opaque, and visual inspection of the electronic components underneath the paint is difficult if not impossible.
Consequently, there is a need for an improved approach to the protection of electronic structures. The present invention fulfills this need, and further provides related advantages.
The present invention provides a conformally coated structure wherein the conformal coating encapsulates the structure and protects it against mechanical and chemical damage. The conformal coating also protects the structure against the buildup of static electrical charges, but also provides shorting protection. Its electrical resistivity remains nearly constant over a wide range of temperatures, an important consideration for aircraft and spacecraft conformal coatings. The conformal coating has excellent mechanical integrity, substantially the same as that of conventional (nonconductive) conformal coatings, and is readily applied. There is no danger of shorting between closely spaced structures through the conformal coating. The protection against static charges is accomplished at low cost by the application of the conformal coating, reducing the labor costs associated with conventional grounding procedures. The conformal coating may be reworked after application by solvent removal.
In accordance with the invention, a conformally coated structure comprises a structure and a conformal coating applied to the structure. The conformal coating comprises a polymer blend of a base polymer and an electrically conductive polymer. The structure typically comprises a component of an electronic assembly such as a microelectronic device, a part of an electronic package, or an electrically conductive trace. The conformal coating is preferably applied in an uncured state, and then cured. The electrically conductive polymer is preferably a polyaniline such as an acid-doped polyaniline, and the base polymer is preferably a urethane, an epoxy, a silicone, or an acrylic.
The electrical resistivity of the polymer blend is preferably from about 109 to about 1013 ohm-centimeter. This resistivity is sufficiently high to prevent shorting between adjacent components and between components and external objects. This resistivity is sufficiently low to permit accumulated static electrical charges deposited upon the surface of the structure to be slowly conducted to ground in a carefully controlled discharge, before they can accumulate to such a degree that there is danger of an uncontrolled discharge event such as an arc or other disruption of the protected electronic components.
The value of the electrical resistivity of the conformal coating is determined by the relative proportions of the electrically conductive polymer and the base polymer. A blend containing about 0.5 percent to about 4 percent by weight of the electrically conductive polymer, with the remainder of the blend comprising the base polymer, results in an electrical resistivity that is suitable.
The electrical resistivity is nearly constant over a wide range of temperatures, including the temperature range of xe2x88x9280xc2x0 C. to +80xc2x0 C. encountered in many spacecraft applications. They may accomplish this constancy of electrical resistivity only with the present approach. The use of polymers filled with metal oxide, carbon, and/or other particulate fillers results in a much wider variability of the electrical resistivity of the conformal coating over such a temperature range. In the particle-filled conformal coatings, the conformal coating is more conductive at colder temperatures as the coating shrinks and particles become closer together, and less conductive at higher temperatures as the coating expands and the particles move farther apart. The polymeric filler of the present invention resists these changes in electrical resistivity as the temperature changes.
The low percentage of the electrically conductive polymer in the blend has the important consequence that the mechanical and protective properties of the conformal coating are essentially those of the base polymer. The conformal coating is therefore applied by the same techniques as used for the base polymer and cured substantially as recommended for the base polymer. The integrity of the conformal coating is substantially the same as that of the base polymer. The conformal coating protects the coated structure from mechanical and chemical intrusion as does the base polymer. Thus, the conformal coating of the invention has properties like those of a conformal coating of the base polymer, but with the added benefit of reducing static charges before they can accumulate to the degree that an uncontrolled discharge may occur or there is other damage to electronic components.
The present approach also offers the opportunity for inspection of the underlying structure through the conformal coating in some circumstances. At a conductive polymer concentration of from about 0.5 percent to about 1.0 percent by weight, the conformal coating is sufficiently transparent (i.e., translucent) to permit inspection of the underlying structure, such as the solder joints. At higher concentrations of the conductive polymer above about 1.0 percent, the conformal coating becomes too opaque to permit such inspection.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.